pH crash...too much CO2 off DIY?

So i noticed my school of rasboras hanging out towards the top of the tank more than usual today (but not abnormally so) so i figured why not test the water. this is a well established, well planted tank so i hadn't tested the water in months. i took out my handy laboret drop test kit and decided to do a ph test first. Came up with a ph of 5. I didn't want to believe my ph could be that low so i tested the tap which also tested at 5...now i knew something was wrong. Went ahead and checked gh and kh, and both showed 1. Maybe the droppers were dirty, so i washed them under hot water for a few minutes, then retested ph. Tap came up at 7 (as it always has), but tank water is still at 5! I'm going to wash the droppers overnight and try again but...

I have been dosing co2 for about a year in this tank without issues (always test at around 7). i've been using the same 1 liter bottle with the same formula the entire time (1c sugar, 1/4 tbs yeast, rest with warm water) and the only change has been putting the outlet of the reactor straight into the filter inlet (Rena XP1). I did a 50% water change yesterday just as I do every week. Could my ph really have dropped that much in 24 hrs? By my calculations that would put me at 300ppm of co2!!!!! I have unhooked the reactor for now to see what happens and i'll be doing another 50% water change. There has been no changes in behavior to any other tank inhabitant (gouramis, kuhlis, or the temporary ottos). Any ideas?

no peat (though it had been considered since its a biotope tank) but yes there is bogwood. 3 of the pieces have been in there since the tank was first set up a year ago, and the other piece has been in for about 2 months but was soaked and boiled before that.

25-50% weekly changes...last one i did was 50% about 2 days ago.